In mammals, the progress of spermatogenesis is subject to temporally and spatially specific translational regulation. But it is not known how specific transcripts are chosen or how they interact with effectors of translation. This project bridges the gap with new genetic resources. Our previous work demonstrated that a eukaryotic translation initiation factor, EIF4G3, is required for cell cycle progression in spermatogenesis and male fertility. Surprisingly, EIF4G3 exhibits a high level of translational selectivity; a specfic EIF4G3 substrate, the Hspa2 mRNA, encoding the heat shock chaperone protein HSPA2, is a reporter transcript for this study of selectivity mechanisms. The male infertility phenotype of Eif4g3 mutant mice provides compelling evidence for the crucial importance of translational selectivity in spermatogenesis. The genetic model provides invaluable resources to delineate mechanisms by which specificity of translational regulation is achieved. Experimental approaches of this project will address the roles of both the reporter transcript (Hspa2) and the effector protein, EIF4G3, that regulates its translational fate in spermatogenesis. Aim 1 will investigate the mRNA substrate that is subject to selective translation by using genetic models and protein interaction methods to determine both transcript sequence features and the RNA binding proteins (RBPs) that are crucial for its translational activation by EIF4G3. Aim 2 will investigate the regulator of translational specificity, EIF4G3, by using new genetic models and cell biological analyses to determine the domain features that allow it to regulate translation in the context of cell cycle progress during spermatogenesis. Aim 3 will address the extent of the specificity mechanism in spermatogenesis by using genetic resources to identify other substrates subject to similar translational regulation by EIF4G3. Together, these aims will define the network of players and resolve mechanisms that enforce the remarkable translational selectivity so crucially required for successful spermatogenesis. !